The metabolism of [the tranquilizer] propionylpromazine in the horse was studied. Although propionylpromazine is not currently approved or recommended for use in horses, it has been used illegally to alter their performance. Propionylpromazine hydrochloride was administered i.m. at clinical and subclinical doses. Three metabolites were detected in urine. The major metabolite was identified as 2-(1-hydroxypropyl) promazine sulfoxide. The detection of this metabolite in routine drug testing is described.